The apparatus disclosed herein involves certain novel, useful, and unobvious screw improvements to a machine used to convert organic material into a char material through a process of carbonizing or pyrolysis.
Conversion of municipal solid waste by incineration has become objectionable due to the release of fumes and smoke that contribute to pollution. In addition, incineration plants typically require additional inputs of expensive energy yet fail to recover useful products that could offset the cost of incineration. Burying municipal solid waste in landfill areas is also objectionable due to the tremendous volumes of waste generated and the scarcity of landfill areas.
Municipal waste and other biomass may be converted to energy and useful products by a carbonizing process as disclosed, for example, in U.S. Pat. No. 5,017,269 to Loomans et al, which is incorporated by reference for what it teaches about the process and the prior art equipment. Organic material is subjected to a sequence of mechanical compressions, intensive mixing, and decompressions in a continuous, twin screw reactor under adiabatic conditions. The intensive mixing subjects the organic material to frictional and viscous shear forces that create heat build up and particle attrition sufficient to change the phase of the particles and to convert their form. This is in contrast to extensive mixing which merely creates a most homogenous distribution of neat ingredients without changing them or converting their form. As a result of the carbonizing process, the organic material gives up volatile hydrocarbons, which may be captured or combusted immediately to provide energy to power the conversion apparatus.
Further, at the end of the process, the organic material is completely converted into a char material of exceptional quality that may be sold as a replacement for, or supplement to, high-grade coal.
Referring now to FIGS. 1 through 2, an exemplary carbonization machine is shown in greater detail. FIG. 1 shows the exemplary carbonizing machine 400 in cross-sectional view. FIG. 2 shows a top view of the carbonizing machine. FIGS. 1 and 2 only show a portion of the overall carbonizing machine 400, focusing on the portion of the machine from an organic material input hopper 450 to just beyond a third gas vent 460. It should be understood by reference to the Loomans et al patent set forth Supra, that the carbonizing machine 400 will further include an opening to output converted char material, among other features.
Referring now particularly to FIG. 1, the carbonizing machine 400 includes dual screws 424 in a chassis 420. The dual screws 424 extend through the chassis 420 by traversing a series of barrel sections 428, 432, 436, 440, 444, and 448. The dual screws 424 take on several distinct, complex, and complimentary screw and paddle designs and orientations. Generally, the dual screws 424 are designed and oriented to perform the tasks of (1) moving the organic material in a leftward direction from the input hopper 450, (2) imparting work energy into the organic material through shredding, chopping, grinding, intensive mixing, and compression to thereby heat the organic material, (3) maintaining near adiabatic conditions, and (4) staging the outflow of gases from the organic material to thereby perform a controlled and highly repeatable conversion into char.
Immediately downstream of the second barrel section is a third barrel section. Here, the dual screws are helical advancing screw design as in the first barrel section. As a result, the organic material advances more rapidly and is, therefore, decompressed. By allowing the material to “relax,” it does not flow out of a second opening in the chassis. Immediately downstream of the third barrel section is a fourth barrel section. The dual screws transition to helical screw sections of decreased pitch such that the organic material begins to recompress. The dual screws then transition to, for example, radially abutting paddles that are progressively axially angularly out of phase or offset to the screw shaft. As a result, the organic material is further chopped, densified, intensively mixed, and compressed. By this point in the process, the organic material typically takes on a dark brown color and reaches a temperature in excess of 450 degrees F.
At the downstream end of the fourth barrel section, the dual screws abruptly transition to reverse hand such that the forward flowing organic material meets itself in reverse flow. As a result, the material is effectively worked against itself to significantly increase the work energy input and to highly compress the material. The material is heated sufficiently to drive off lighter volatiles (hydrocarbons). However, with the organic material so highly compressed, another vapor block is formed at the downstream end of the fourth barrel section. Therefore, the lighter volatiles flow back upstream into the third barrel section. The lighter volatiles are released, as the second vented gas, through the second opening in the chassis. A second duct vents the second gas, which may be combusted, to provide energy to drive the dual screws or it may be condensed for other uses.
Organic material, such as municipal solid waste (MSW), is shredded, ground, and dried before it is fed into a first barrel section 428 of the machine 400 through a feed port hopper 450. In the first barrel section 428, for example, the dual screws 424 are helical with lenticular cross-sections and primarily designed to advance the organic material.
Immediately downstream of the first barrel section 428 is a second barrel region 432. In the second barrel region 432, the dual screws 424 transition to, for example, radially abutting paddles that are progressively axially angularly out of phase or offset to the screw shaft. In addition, axially adjacent paddles on each shaft are arranged in helical formation. Toward the downstream end of the second barrel section 432, the dual screws incorporate a reverse helical formation to exert a counter-stream flow force on the material. This counter-stream force causes the material to be further compressed.
As the organic material is chopped, intensively mixed, and compressed in the second barrel section 432, the material is heated by work energy. As the organic material traverses the second barrel section 432, it is heated to about 400 degrees F. Although the organic material is dried prior to input into the machine 400, it still typically includes residual moisture. The work energy heating in the second barrel section 432 is sufficient to drive off moisture as steam in the second barrel section 432. The helical and reverse helical designs in the dual screws 424 of the second barrel section 432 cause the organic material to become so highly compressed at the downstream end of the second barrel section 432 that a vapor block is formed. Therefore, the emitted steam is forced back upstream into the first barrel section 428. The steam is released as the first vented gas 466 through a first opening 454 in the chassis 420. A first duct 452 vents this steam 466, which may be routed to a heat recovery process.
Immediately downstream of the second barrel section 432 is a third barrel section 436. Here, the dual screws 424 return to a helical advancing screw design as in the first barrel section 428. As a result, the organic material advances more rapidly and is, therefore, decompressed. By allowing the material to “relax,” it does not flow out of a second opening 458 in the chassis 420. Immediately downstream of the third barrel section 436 is a fourth barrel section 440. The dual screws 424 transition to helical screw sections of decreased pitch such that the organic material begins to recompress. The dual screws 424 then transition to, for example, radially abutting paddles that are progressively axially angularly out of phase or offset to the screw shaft. As a result, the organic material is further chopped, densified, intensively mixed, and compressed. By this point in the process, the organic material typically takes on a dark brown color and reaches a temperature in excess of 450 degrees F.
At the downstream end of the fourth barrel section 440, the dual screws 424 abruptly transition to reverse hand such that the forward flowing organic material meets itself in reverse flow. As a result, the material is effectively worked against itself to significantly increase the work energy input and to highly compress the material. The material is heated sufficiently to drive off lighter volatiles (hydrocarbons). However, with the organic material so highly compressed, another vapor block is formed at the downstream end of the fourth barrel section 440. Therefore, the lighter volatiles flow back upstream into the third barrel section 436. The lighter volatiles are released, as the second vented gas 470, through the second opening 456 in the chassis 420. A second duct 456 vents the second gas 470, which may be combusted, to provide energy to drive the dual screws, for example, or condensed for other uses.
Immediately downstream of the fourth barrel section 440 is a fifth barrel section 444. Here, the dual screws 424 return to a helical advancing screw design as in the first and third barrel sections 428 and 436. As a result, the organic material is again briefly decompressed to prevent out flow at a third opening 462 in the chassis 420. Immediately downstream of the fifth barrel section 444 is a sixth barrel section 448. The dual screws 424 again transition to helical screw sections of decreased pitch to cause recompression and heating. The dual screws 424 then transition, for example, to reverse hand to input significant work energy and highly compress the material. The material is heated sufficiently to drive off heavy volatiles (hydrocarbons). Another vapor block forms at the downstream end of the sixth barrel section 448 and forces the heavy volatiles back upstream into the fifth barrel section 444. The heavy volatiles are released, as the third vented gas 474, through the third opening 462 in the chassis 420. A third duct 460 vents the third gas 470, which may be routed to a combustion chamber. At the end of the sixth barrel section 448, the organic material reaches a temperature of about 600 degree F. At this point, the organic material is completely black and bears a charcoal-like appearance.
The oxygen-free environment prevents the material, converted now to char, from igniting. A final cooling process is typically performed before the char is removed. As described above, the carbonizing machine 400 is specifically designed to prevent outflow of the solid organic material at the first, second, and third openings 454, 458, and 462. However, two types of problems are found to occur in the gas openings 454, 458, and 462 and the gas ducts 452, 456, and 460. First, the incoming organic material is very dry and not very dense. It is therefore commonly called fluff. It is found that steam backflow from the second barrel 432 can carry some of the organic material fluff up the first opening 454 along with the steam 466. The fluff then deposits on the sidewalls of the first opening 454 or the first duct 452. The deposited fluff is a brownish, fluffy layer that can obstruct the first opening 454 and first duct 452 if not removed.
Second, as described above, the lighter and heavy volatile oils are released from the second opening 458 and the third opening 462, respectively, as the second gas 470 and third gas 474. The lighter volatiles are typically heated to between about 380 degrees and 400 degrees F. The heavy volatiles are typically heated to between about 520 degrees and 540 degrees F. The ambient temperature surrounding the machine 400 and the second and third ducts 456 and 460 is much lower. It is found that a portion of the gases 470 and 474 condenses on the sidewalls of the second and third openings 458 and 462 and on the second and third ducts 456 and 460. When the volatiles condense on the sidewalls of the openings and the ducts, any entrained particulate matter (such as the finely ground organic material) will easily stick to the sidewalls. As a result, the condensed lighter volatile matter forms a black, crusty layer. The condensed heavy volatile forms a black grease. Either deposit can obstruct the second or third opening 458 and 462 and second or third duct 456 and 460 if not removed.
Referring now to FIG. 2, a top view of the exemplary carbonizing machine 400 is shown. Portions of the dual screws 424′ and 424″ may be seen in this Figure. The dual screws 424′ and 424″ run the length of the machine 400. The hopper 450 is located over the dual screws 424′ and 424″ so that inputted organic material is captured and then transferred downstream by the screws. The first vent 452 is located over the dual screws 424′ and 424″ toward the end of the first barrel section. The second vent 456 is located over the dual screws 424′ and 424″ in the third barrel section. The third vent 460 is located over the dual screws 424′ and 424″ in the fifth barrel section.
Referring now to FIG. 3, a cross-sectional view of a carbonizing machine 414, taken through line A-A of FIG. 1, shows the material deposition problem as found in the prior art. An opening 484 is positioned to release gas 486 from a barrel. However, material 480 is deposited on the sidewalls of the opening 484 in the chassis 420 or on the duct 482. This deposited material 480 may be the organic material fluff carried by released steam, as in the first opening of the carbonizing machine 400 of FIGS. 1 and 2, or condensed volatile and particulate matter, as in the second and third openings of the carbonizing machine 400 of FIGS. 1 and 2. Preventing the escape of these materials into the vents and preventing buildup of deposited material 480 is a pressing and unmet need in the art.
Looking at the typical cross section of a bi lobed twin screw mixer screw profile (FIG. 4), we discover that the cross section is typically a football shape where in one of the co-rotating twin screw bi-lobe geometries is described by the mating screw as they are conjugal as described in an earlier U.S. Pat. No. 3,195,868_B. Loomans et al and U.S. Pat. No. 3,198,491_B. Loomans et al, Example #1.
One problem with the current design of the system described in U.S. Pat. No. 5,017,269 to Loomans et al, is the transition from the free flowing geometries to those that create the compression and work energy that goes into the materials. These transitions are abrupt and create dams in the continuous processor that typically obstruct the forward flow of material and materially contribute to the back flow of particulate solids back into the venting area.
There are two conditions that promote the carry over and build up of particular solids in the feeding and venting areas. One of them is the velocity of the gas flow out the barrel openings (which if too high carries solids out with the gas phase) and the second is the restriction of forward flow of solid material down the barrel from the vents by abrupt changes in the screw geometry where the material transitions from a relaxed state to a compressed state (causing solid material to back up into the vent areas).